Circuit switched (CS) fallback (CSFB) in an evolved packet system (EPS) enables the provisioning of voice and other CS domain services when a wireless transmit/receive unit (WTRU) is served by an evolved universal terrestrial radio access network (E-UTRAN). FIG. 1A shows a conventional EPS architecture 100 including a CSFB-capable WTRU 105, an E-UTRAN 110, a mobility management entity (MME) 115, mobile switching center (MSC)/visitor location register (VLR) 120, a (GSM)/enhanced data rates for GSM evolution (EDGE) radio access network (GERAN) 125, a UTRAN 130 and a serving general packet radio service (GPRS) support node (SGSN) 135. The WTRU 105 may include at least one timer 140. The MME 115 communicates with the MSC/VLR 120 via an SGs interface 145. CSFB and Internet protocol (IP) multimedia subsystem (IMS)-based services, such as a short messaging service (SMS), may coexist in the conventional EPS architecture 100. The CSFB-capable WTRU 105 may use the GERAN 130 or the UTRAN 125 to connect to the CS domain, and use the E-UTRAN 110 to connect to a packet switched (PS) domain (e.g., a long term evolution (LTE) network), for providing IMS voice services. The conventional EPS architecture 100 of FIG. 1A enables the WTRU 105 to use the PS domain to receive CS services, such as SMS, as well as place a CS call. However, other services, such as CS voice and location services, require that the CS domain be used for the purpose of actual exchange of voice or location service data. In order for the WTRU 105 to have access to both the PS and CS domains, the WTRU 105 performs a combined EPS/international mobile subscriber identity (IMSI) registration procedure.
FIG. 1B is a signal flow diagram of a CSFB call establishment procedure performed in the conventional EPS architecture 100 of FIG. 1A. When the WTRU 105 is registered to both the PS and CS domains after sending an attach message or a tracking area update (TAU) message, the WTRU 105 may be combined EPS/IMSI attached (150). The WTRU 105 may initiate a CSFB request procedure by sending a mobile originated (MO) CSFB extended service request (ESR) message to the MME 115 (155) and activating a timer 140, (i.e., T3417ext), in the WTRU 105 (160). The timer 140 may be set to a predetermined time period, (e.g., 10 seconds) to implement the CSFB request procedure. If the CSFB request procedure succeeds, the timer 140 is deactivated prior to expiration (162), (e.g., set to a value of zero). Otherwise, the WTRU waits for the timer 140 to expire on a condition that the CSFB request procedure is aborted (164). The problem with the CSFB call establishment procedure of FIG. 1B is that the amount of time that the WTRU 105 has to waste waiting to find out that the CSFB request procedure has failed is unacceptable. If the CSFB request procedure fails, a user of the WTRU 105 ends up having to send another ESR message (166) or take alternative action, which wastes time.
FIG. 1C is a signal flow diagram of a CSFB call establishment procedure performed in the conventional EPS architecture 100, whereby access to the CS domain is temporarily restricted due to congestion or the implementation of an operator policy. When the WTRU 105 is registered to both the PS and CS domains after sending an attach or TAU message, the WTRU 105 is combined EPS/IMSI attached (170). The WTRU 105 may initiate a CSFB request procedure by sending an MO CSFB ESR message (175) to the MME 115. Due to the temporarily restricted access to the CS domain, the MME 115 may send a service reject message (180) to the WTRU 105 to indicate, using a cause value, (e.g., #39), that the CS domain will temporarily not be available for a predetermined period of time, as defined by a timer value in the service reject message. The timer 140, (i.e., T3442), in the WTRU 105 may be set to the timer value included in the service reject message and activated (182). The WTRU 105 has to wait for the timer 140 to expire (184) before being able to send another ESR message (186).
However, as shown in FIG. 1D, the MSC/VLR 120 may receive a mobile terminated (MT) call request for the WTRU 105, and request the MME 115 to notify the WTRU 105. The MME 115 may send a paging request (190) to the E-UTRAN 110 to page the WTRU 105 (192) for an MT CS call. The WTRU 105 may respond to the page with an ESR message for an MT CSFB call (194). However, the timer 140 continues running until it expires, thus indicating that the CS domain is congested, even though that may no longer be the case. The MME 115 may then send redirection/handover information, (e.g., parameters for a target cell), to the WTRU 115 (196).
The problem with the CSFB call establishment procedures of FIGS. 1B, 1C and 1D is that there is no provision for expediting the handling of CSFB emergency calls when the CS domain is congested.